1. Field of the Invention
The present invention relates to a megakaryocyte differentiation factor, gene coding for the factor and a process for production thereof. The megakaryocyte differentiation factor is useful as a hemopoietic stimulating factor for megakaryocyte-platelet lineage.
2. Description of Related Art
It is well known that various hemopoietic factors inducing the growth and differentiation of blood cells are involved in a process from hemopoietic stem cells to mature blood cells.
Although the life time of platelet is as short as 9 to 10 days, concentration of platelets in the blood is maintained rather constant during the stationary state. Moreover, when the number of platelets is artificially reduced by one of various available method in an experimental animals, the number of the platelets recovers in the blood in a few days. From these facts, it is supposed that factors which stimulate formation of platelets are present, and so far a great effort has been made to identifycation of the factors.
It is considered that at least two regulatory factors are involved in the megakaryocyte-platelet hemopoietic lineage. The first factor by itself stimulates formation of megakaryocyte colonies and is called a megakaryocyte colony stimulating factor. The second factor by itself does not have an activity to stimulate formation of megakaryocyte colonies, but in combination with the first factor, increases the number of megakaryocyte colonies and stimulates the differentiation thereof, and is called a megakaryocyte potentiator.
The former includes interleukin 3, and granulocyte/macrophage colony stimulating factor, and the latter includes erythropoietin, macrophage colony stimulating factor, interleukin 6, 7 and 11, LIF, and the like. Some of these factors actually exhibit in vivo effect of increasing the number of platelets or shortening the time required to recover the number of platelets (Hideaki Mizoguchi: Tanpakushitsu Kakusan Koso 36, 1195, 1991 in Japanese).
However, most of these factors exhibit a diversity of biological activities other than participation in differentiation of blood cells in various hemopoietic lineages including differentiation in megakaryocyte-platelet lineages. For example, although IL-6 and IL-11 actually exhibit in vivo thrombopoietic action, they stimulate production of acute phase protein, and in severe cases, cause cachexia. Moreover, IL-6 is accompanied with various clinical problems; for example, it is possible for IL-6 to stimulate the growth of mesangium cells in the kidney resulting in renal failure (Tadashi Matsuda et al., Tanpakushitsu Kakusan Koso, 36, 1184, 1991 in Japanese). In addition, since IL-6 does not exhibit a high blood level during a thrombcytopenic phase, it is not considered as a physiological factor.
Platelets play an important role in a hemostatic mechanism. Diseases involving decrease of platelets (Fanconi's syndrome, megakaryocytic thrombocytopenia, aplastic anemia, and the like) are clinically dangerous, and in particular hemorrhaging cannot be controlled. Therefore, it is considered that isolation and identification of a factor which stimulates production of platelets is useful to prevent the above-mentioned danger.
Currently, bone marrow transplantation is becoming a powerful therapeutic means for treating leukenia etc., and the ratio of successful cases is increasing through the use of cytokines such as erythropoietin (EPO), granulocyte colony stimulating factor (G-CSF) etc. At present a problem in the bone marrow transplantation is a decrease in the number of platelets, and if a thrombopoietic factor is available, it is expected that the ratio of successful cases will increase and a period of hospitalization will be shortened. Not only hemopoietic diseases but also thrombocytopenia in chemotherapy and radio isotopic therapy of cancers may be controlled by thrombopoietin.
The present inventors, considering the various above-mentioned difficulties with known factors, carried out various research to find a factor which stimulates production of platelets and is effective for treatment of patients having thrombocytopenia or insufficient platelet function, and as a result, the present inventors found a novel factor which stimulates differentiation of megakaryocytes, cloned a gene coding for said factor, constructed an expression vector, and succeeded in expressing the gene to produce said factor.